In recent years, many studies have been actively made on an organic thin-film light emitting device that emits light upon recombination of electrons injected from a cathode and holes injected from an anode in an organic light emitting body interposed between both the electrodes. The light emitting device has been attracting attention because the device is thin and emits light with high luminance under a low driving voltage, and multi-color emission can be obtained by selecting light emitting materials.
Since an organic thin-film light emitting device with high luminance emission has been reported by C. W. Tang of Kodak, many studies have been made by research facilities. The typical organic thin-film light emitting device reported by a Kodak research group is successively constructed by an ITO glass substrate, a hole transporting layer including an diamine compound, a light emitting layer including tris(8-quinolinolato)aluminum(III), and a Mg:Ag cathode and emits green light with a luminance of 1,000 cd/m2 at a driving voltage of about 10 V (Non-Patent Document 1).
Many researches have been made on the applications of the organic thin-film light emitting device to display, etc. because of its possibility of a wide selection of emission colors by using various emitting materials in the light emitting layer. Particularly, the research on the materials which emit three primary red, green, blue colors has been made most actively, and the intensive research has been made to improve their properties.
One of the most important problems in the organic thin-film light emitting device is to achieve both high emission efficiency and low driving voltage. Patent Document 1 reports that a highly efficient light-emitting device is obtained by forming a light emitting layer wherein a several percent of a dopant material is doped to a host material. The host material is required to have a high carrier mobility and a uniform film-forming property, and the dopant material is required to have a high fluorescent quantum yield and a uniform dispersibility.
A fluorescent (emission from singlet state) material has been generally used as the dopant material. To improve the emission efficiency, the use of a phosphorescent (emission from triplet state) material has been studied, and a Princeton University group has demonstrated that the phosphorescent material drastically improved the emission efficiency as compared with the known fluorescent material (Non-Patent Document 2). As the phosphorescent dopant material, metal complexes having a central metal, such as iridium, osmium, rhodium, palladium and platinum, have been disclosed (Patent Documents 2 to 4). As the host material to be combinedly used with the phosphorescent dopant material, carbazole derivatives, aromatic amine derivatives, quinolinol metal complexes, etc. have been disclosed (Patent Documents 2 to 6). However, a device achieving both a sufficient emission efficiency and a low driving voltage is not obtained by the known host material.
The host material, the hole transporting material, and the electron transporting material existing around the phosphorescent dopant in an phosphorescent organic electroluminescence device are required to have a high triplet excited energy, i.e., have a wide gap capable of energetically confining the excitons of emitting dopant for achieving a high internal quantum efficiency; a high carrier injection and transport ability for achieving a high power conversion efficiency to allow a driving at low voltage; and a high chemical and heat stability for achieving a long lifetime. The device performance may be optimized by optimizing the chemical structure of material. If the performance (mainly carrier balance) can be simply controlled by modifying the chemical structure of material, the study on the material can be significantly facilitated.
However, the material for organic electroluminescence device, such as the host material, the hole transporting material and the electron transporting material, which meets the above requirements is not yet reported.
Patent Documents 7 to 9 disclose compounds having a carbazole ring, etc. at the terminal end.